Fourier transform time of flight and ion mobility (and other velocity dispersive analysis) are known. For example, see Knorr et al Anal. Chem. 1986. 58. 690-694 and Knorr, Hill Anal. Chem. 1985, 57, 402-406.
Fourier Transform Ion Mobility Spectrometry (FT-IMS) is a multiplexing technique in which ions are gated into and out of the ion mobility separator (IMS) cell by ion gates arranged at the ends of the IMS device. The gating signal that opens and closes the ion gate is generally identical on both ion gates and the frequency of the gating signal on each ion gate is swept with time. The duty cycle of the gating of the ion beam on both gates is generally set to 50%.
According to this arrangement, the amplitude of the output signal from the IMS device, for an ion of specific mobility, varies as a substantially triangular function that has a frequency that is characteristic of the ion's ion mobility. The ion signal may be measured as a function of the ion gate signal frequency. A Fourier transform is then applied to this data obtained in the ion gate modulation frequency domain so as to produce an ion mobility separation spectrum.
An advantage of FT-IMS is a much improved duty cycle compared to conventional atmospheric pressure ion mobility separation. In conventional atmospheric pressure ion mobility separation, ions are introduced into the drift region by rapidly opening and closing an ion gate once per IMS separation cycle. Typical gating times are in the order of 100 micro-seconds, whereas typical drift times through the IMS device are in the order of 100 milli-seconds, thus leading to a duty cycle in the order of 0.1%.
FT-IMS also has advantages over sub-atmospheric RF confined IMS techniques. In sub-atmospheric RF confined IMS devices ions are intermittently pulsed into the IMS device. In order to improve the duty cycle, between pulses when ions are not being admitted into the IMS device, the ions may be accumulated in an ion trapping region upstream of the IMS device. However, if the ion flux towards the IMS device is high then the charge density in the ion trapping region, or in the IMS device, may become high and the resulting space-charge effects may cause a loss of signal or distortions in the drift times of ions through the IMS device. These problems may be avoided in FT-IMS techniques because gating frequency allows the device to receive a continuous ion beam and operate with a relatively high duty cycle, without the need to store ions in an upstream ion trap.
Fourier transform techniques are also known to be used in orbital trapping electrostatic ion traps and FT-ICR mass spectrometers. According to these techniques, ions oscillate in a trapping field in a manner that is dependent on the mass to charge ratios of the ions. These oscillations are detected and the resulting signal is measured in the time domain. This signal is then converted to the frequency domain by using a Fourier transform to produce a mass spectrum. In these instruments, Fourier transformation of time domain data results in a complex frequency spectrum (i.e. comprising real and imaginary parts). For example, see Ref. J Am Soc Mass Spectrom. 2011 January; 22(1):138-47 Phase Correction of Fourier Transform Ion Cyclotron Resonance Mass Spectra Using MatLab.
When all signals have zero phase, the transformation from the measurement time-domain to the frequency-domain can be written as follows:F(ω)=∫F(t)eiωtdt=A(ω)+iD(ω)
where ω is the characteristic frequency of the amplitude of the measured signal; F(ω) is the frequency domain data; A(ω) is the real part of the spectrum (absorption mode spectrum); and D(ω) is the imaginary part of the spectrum (dispersion mode spectrum).
In general, however, when signals have non-zero phases the real and imaginary parts of the frequency domain data can contain mixtures of the absorption and dispersion mode spectra. This may result in asymmetrical peak shapes for the real component after the Fourier transformation. In order to avoid such asymmetrical peak shapes it is known to use the phase-independent magnitude spectrum M(ω) of a Fourier transformation in order to determine the frequency and hence mass to charge ratios of the ions. The magnitude spectrum M(ω) is given by:M(ω)=[(A(ω))2+(D(ω))2]0.5 
The magnitude spectrum disregards phase information and so provides symmetrical peaks. However, the magnitude spectrum provides a relatively low resolution spectrum.
It is desired to provide an improved method of ion mobility spectrometry and an spectrometer therefor.